Jimmy Connors was quite a grunter, but Monika Seles was probably the worst grunter or squealer of all times. So you have to go quite a few years back...
Sorry SB ...
Monica Seles gruntometer = 93.2 db
Maria Sherapova = 101 db
http://news.bbc.co.uk/2/hi/uk_news/magazine/4118708.stm